There are a large number of sound sources in an automobile. From the viewpoint of the demand for quietness away from automotive inside and outside noises, various soundproof measures have been taken. In particular, with regard to the components (intrinsic noise sources) that generate loud sounds, such as engines, transmissions and driving systems, soundproof measures are required in the positions near to the sound sources. Thus, a dedicated soundproof cover excellent in sound-absorbing and insulating performance is used. The demand for noise-reducing components in automobiles is very high, combined with that automotive outside noise level regulations are tightened by a series of legal changes and that a reduction in automotive inside noise is directly linked to a car value (a touch of class). In particular, an automotive outside noise regulation scheduled to be introduced in the European Union in 2013 is finally as severe as −3 dB to the conventional regulation value (it is necessary to be reduced to one half in terms of sound pressure energy). This essentially requires noise reduction measures against the intrinsic noise sources such as basic engines as main noise emitting sources in engine rooms and transmissions. Although various soundproof components such as engine top covers on the side of upper surfaces of engines have hitherto been used, however, further improvement in performance and weight saving from the viewpoint of a decrease in fuel consumption have been demanded.
Conventional soundproof covers are designed with putting the principal objective thereof to insulation of direct noise from the intrinsic noise sources, and are each constructed so that a sound-absorbing material is post-attached to the intrinsic noise source side of a rigid cover formed by molding a metal or a resin such as polyamide or polypropylene, or to a part thereof (see Patent Document 1). However, the sound-insulating performance of such a soundproof cover conforms to the mass law, and depends on the weight of the rigid cover. It is therefore impossible to comply with the needs for weight saving. Further, in the case where the intrinsic noise source is accompanied by vibration, even when the vibration is transmitted from fixing points and the like for attaching the soundproof cover to the engine and the like, the rigid cover hardly undergoes oscillatory deformation, and hence an effect of damping the vibration as kinetic energy is not obtained. Accordingly, secondary emission occurs from a rigid noise insulating layer to deteriorate the noise level rather than to improve it in some cases.
Moreover, regarding the evaluation of noises inside and outside an automobile, since the noise level itself is an amount of sense of human, a sound pressure level (dB) obtained by logarithmically compressing an observed sound pressure is used as an evaluation criterion close to an amount of the sound sensed by human. However, when a four (multi)-directional average (combination sound) which is generally employed in a case of evaluating a general soundproofing effect (the increase or decrease in sound pressure level) is calculated, the sound pressure level is affected by the largest sound of all the measured sounds because of the characteristic of the dB sum calculation. Therefore, even though the sound pressure level in only one direction in which a sound proof measure has been taken is reduced, the soundproof effect could not be attained as a whole with the result that the sound pressure level as the level of human sense to sound could not be lowered in some cases. Accordingly, it is necessary to thoroughly and uniformly reduce the sound pressure level in every direction.
However, with the soundproof cover disclosed in Patent Document 1 constructed by attaching a sound-absorbing material to a rigid cover, the rigid cover may be resonant with vibration transmission (solid-borne sounds) in case where the intrinsic noise sources vibrate, thereby generating noises by itself to be a new noise emitting source (secondary emission). In general, therefore, the soundproof cover must be fixed to the intrinsic noise sources via a vibration-insulating material such as rubber bush. Necessarily, therefore, there occurs a gap between the peripheral edge of the soundproof cover and the intrinsic noise source, and there may be caused where inner reverberationg sounds (standing waves) leak out from the gap and the sound level reduction cannot be attained.
From such a background, for the purpose of taking measures against solid-borne sounds in the case where the intrinsic noise sources vibrate or inner reverberating sounds (standing waves) of a soundproof cover, there has been proposed a soundproof cover, in which a soft sound-insulating layer formed of a nonwoven fabric coated with a vibration-damping resin is provided, in place of a rigid cover, on a surface of a sound-absorbing material on the opposite side of an intrinsic noise source (see Patent Document 2).
However, the soundproof cover described in Patent Document 2 has a limitation in its mass from a manufacturing problem of the soft sound-insulating layer, and is inferior in sound-insulating performance in a high-frequency region of 4 kHz or more to the high-mass rigid cover.    [Patent Document 1] JP-A-10-205352    [Patent Document 2] JP-A-2006-98966